1. Field of the Invention
The present invention relates to a rotation detecting apparatus for detecting the rotation of various machines and equipments such as, for example, for detecting the rotation for controlling rotation of a compact motor, or for detecting the rotation for detecting the position within a business equipment, and also to a bearing assembly equipped with such rotation detecting apparatus.
2. Description of the Prior Art
Taking advantage of the compactness and the easiness of assemblage, a bearing assembly having a rotation sensor built therein has hitherto been suggested. One example of such bearing assembly is shown in FIG. 29. In this example, a rotating ring 52 of a rolling bearing assembly 51 is fixed with a member including a magnetic encoder 54 in the form of rubber magnets having a plurality of alternating magnetic poles N and S deployed in a circumferential direction thereof, and a stationary ring 53 is fixed with a member including a magnetic sensor 55 such as, for example a Hall element or the like so that rotation pulse signals and/or direction of rotation can be obtained.
However, in the structure in which the magnetic encoder 54 is provided in the manner as hereinabove described, in the case of a small diameter bearing, which is a rolling bearing assembly of a small size, problems have been recognized in that it is difficult to allow the magnetic sensor 55 to be accommodated within the outer diametric size of the stationary ring 53 and, also, to detect the rotation angle with high precision enough to secure the number of rotation pulses that is equal to or greater than 500 per each complete rotation.
In view of the foregoing, as a rotational angle detecting device capable of being incorporated in a compact machine and also cable of providing an output descriptive of the rotation angle with high precision, a device utilizing a sensor array has been suggested. (See, for example, the Japanese Laid-open Patent Publication No. 2003-148999 published May 21, 2003.) Such device is of a structure, in which a sensor array comprised of a plurality of magnetic sensor elements is integrated on a sensor chip together with a signal amplifier circuit, an analog-to-digital (A/D) converter circuit and a digital signal processing circuit, which sensor chip is in turn arranged in face-to-face relation to a magnet head mounted on a rotating member. It is the principle of this method that the magnetic sensor array detects a distribution of magnetic fields generated by the magnetic head arranged in face-to-face relation with the sensor chip so that the rotation angle of the magnet can be detected from this distribution. However, in the case of this construction, in a semiconductor circuit, it is unavoidable for circuit elements, integrated on a silicon chip, to result in variation in characteristic and offset variations of the sensor elements tend to occur even in the magnetic sensor array, thus constituting a cause of degradation of the angle detecting precision.
As an improvement to the Japanese Laid-open Patent Publication No. 2003-148999, it has been suggested to arrange the magnetic sensor elements of the sensor array in parallel relation to each other in an attempt to reduce the offset variations for the purpose of reducing the degradation of the angle detecting precision. (See, for example, the Japanese Laid-open Patent Publication No. 2004-037133, published Feb. 5, 2004.) However, even though the magnetic sensor elements are arranged in parallel relation to each other, the residual offset variation still affects the angle detection.
For the magnetic sensor elements referred to above, MAGFET elements are generally utilized, each of which has such a characteristic that when the element is subjected to a vertically acting magnetic field, a non-equilibrium occurs in electric currents flowing across two drain terminals thereof enough to provide a current difference that represents a magnetic signal desired to be detected. However, the current difference generated from the magnetic sensor element is so considerably small as to require amplification thereof.
The magnetic detecting circuit, in which the magnetic sensor elements, i.e., MAGFET elements of the kind discussed above are employed, is reported by Shen-Iuan Liu, Jian-Fan Wei and Guo-Ming Sung in their “SPICE Macro Model for MAGFET and its Applications”, IEEE Trans. Circuits and Systems II, Analog and Digital Signal Processing, vol. 46, 4, 1999, (which is referred to as the non-Patent Document 1). According to this non-Patent Document 1, the current obtained from the terminals of the sensor element is converted into a current difference, which is in turn converted into a voltage signal by the use of an OP amplifier.
However, the circuit disclosed in the non-Patent Document 1 referred to above handles signals on a single-ended system and is therefore susceptible to influence brought about by noises, as well as tends to result in an insufficient amplification factor when the current difference is small. In view of this, the rotation detecting apparatus disclosed in the non-Patent Document 1 referred to above requires the following to be satisfied.
Where a signal from the sensor array is converted into a digital signal in order to calculate the angle, the sensor signal has to be amplified to an amplitude of at least about 1 to 2 volts before it is supplied to an analog-to-digital (A/D) converter. Therefore, a circuit of a structure effective to convert it to a voltage with sufficient amplification factor and operating speed is necessary, resulting in increase of the electric power consumption.
In addition, as a method for canceling the offset of the magnetic sensor elements, the method is known in which the elements are connected parallel to each other, as FIG. 30 shows one example thereof. In the example shown in FIG. 30, two elements 45a and 45b each having two drain terminals D1 and D2 are connected in such a manner that the same drain terminals D1 of those elements 45a and 45b are connected together and the same drain terminals D2 of those elements 45a and 45b are similarly connected together, with connection lines between the drain terminals D1 and between the drain terminals D2 crossing relative to each other.
Those drain currents between the two magnetic sensor elements 45a and 45b, formed on a silicon wafer in juxtaposed relation to each other, are considered ideal if they have an equal amount in the absence of magnetic fields. However, a cant component will occur depending on the manufacturing process, which can result in an offset signal. Specifically, in the presence of this cant component in, for example, a direction rightwards as shown by the arrow A, in each of the magnetic sensor elements 45a and 45b the current will tend to flow rightwards as shown by the respective arrows a and b in FIG. 30. The method in which the drain terminals are cross-connected in the manner shown in FIG. 30 is effective to counterbalance the offset signals occurring in those two sensor elements 45a and 45b. 
However, even where the magnetic sensor elements 45a and 45b are connected parallel to each other as in the example shown in FIG. 30, variation of the residual offset unavoidably adversely affects the precision of angle detection. In particular, in the case where the magnetic sensor elements used is of a native substrate type, a considerable piezoresistive effect brought about by a warp of the silicon chip causes a problem in that the offset of the sensors changes considerably.
It is to be noted that studies on the piezoresistive effect resulting from the warp of the silicon wafer is reported by Yozo Kanda in his “A Graphical Representation of the Piezoresistance Coefficients in Silicon”, IEEE Trans. Electron Device, vol. ED-29, No. 1, January 1982, which is referred to as a non-Patent Document 2, and Jefferey C. Suhling in his “Silicon Piezoresistive Stress Sensors and Their Application in Electronic Packaging”, IEEE Sensors Journal, vol. 1, No. 1, 2001, which is referred to as a non-Patent Document 3. The method of reducing the influence brought about by stresses in Hall elements is reported by, for example, R. Steiner, et al. in their “Offset Reduction in Hall Devices by Continuous Spinning Current Method”, Sensors and Actuators, A66, pp. 167-172, 1998, which is referred to as a non-Patent Document 4.
Specifically, in the sensor element formed in a silicon wafer 40 shown in FIG. 31, the offset component brought about by the piezoresistive effect is caused mainly by a change in resistivity in a 45° orientation relative to the sensor element (according to the non-Patent Document 2). The piezoresistive effect in such case is a phenomenon, in which the electric resistivity in each of X1 and X2 directions of the wafer 40 shown in FIG. 31 changes depending on the stress condition loaded on the wafer 40.
In the circuit configuration in which the magnetic sensor elements are connected parallel as shown in FIG. 30, influence brought about by the stress (shown by S in FIG. 32A) in the silicon chip appears, as shown in FIG. 32A, as a change of the electric resistivity in a direction 45° angled relative to the sensor elements 45a and 45b. Accordingly, the offsets appearing in those two sensor elements 45a and 45b assume the same polarity and are not counterbalanced in this connection. In other words, in FIG. 32A, the resistivity in the rightwardly upward direction (the leftwardly downward direction) becomes lower than the resistivity in a direction perpendicular thereto and, consequently, an imbalanced current flow occurs inside the sensor elements 45a and 45b as shown by the arrows a and b. FIG. 32B illustrates a condition in which a magnetic field Bz is applied while in a condition shown in FIG. 32A. The sensor signal in such case ideally represents an output proportional to the intensity of the magnetic field, but represents a signal superimposed with the stress induced offset.
Also, it has been suggested to determine the rotation angle of the magnet based on an output of the magnetic sensor array. (See, for example, the Japanese Laid-open Patent Publications No. 2004-037133 and No. 2005-043070 published Feb. 17, 2005.) Those publications suggest a rotation detecting device which includes a sensor unit in the form of a sensor array composed of a plurality of arranged magnetic sensor elements and integrated on a semiconductor chip together with a signal amplifying circuit, an A/D converting circuit and a digital signal processing circuit. However, those publications are silent as to a specific type of the magnetic sensor elements.
Xinyu Zheng and Suzhi Wu disclose, in their “General Characteristics and Current Output Mode of a MOS Magnetic Field Sensor”, Sensors and Actuators, A28 (1991), pp-5, which is referred to as a non-Patent Document 5, the fundamental characteristics of the magnetic sensor element MAGFET and indicate the presence of a certain reduction in magnetic sensitivity during the operation at a linear region. However, nothing is mentioned of the offset variation.
James J. Clark has suggested, in his “Split-drain MOSFET Magnetic Sensor Arrays”, Sensors and Actuators, A24 (1990), pp-107-116, which is referred to as a non-Patent Document 6, a method in which the magnetic sensor elements MAGFETs are arranged in a matrix pattern to detect a distribution of magnetic fields. He has also used a read-out circuit based on a simple voltage converting circuit and has described the offset variation of the sensor output being considerable.